Explicitly cast alert data to a list in tests

In lsst/alert_packet#21, we would like to
change lsst.alert.packet's behavior to stream alerts out from disk
rather than loading them entirely into memory, since alert files might
be very large. Technically, this breaks no APIs, since the documented
return value is an "iterable."

Making that change requires this one: change the test code so that it
loads data into a list explicitly.

tests/test_packageAlerts.py

@@ -464,8 +464,9 @@ def testRun(self):
 
         ccdVisitId = self.exposure.getInfo().getVisitInfo().getExposureId()
         with open(os.path.join(tempdir, f"{ccdVisitId}.avro"), 'rb') as f:
-            writer_schema, data = \
+            writer_schema, data_stream = \
                 packageAlerts.alertSchema.retrieve_alerts(f)
+            data = list(data_stream)
         self.assertEqual(len(data), len(self.diaSources))
         for idx, alert in enumerate(data):
             for key, value in alert["diaSource"].items():